Cyclosporine A is an established form of immunosuppressive treatment in organ transplantation involving kidney, liver, or heart. The absorption of cyclosporine during chronic administration of Sandimmune soft gelatin capsules and oral solution is known to be highly variable. Patients taking the soft gelatin capsules or oral solution were monitored at repeated intervals for cyclosporin A levels over a period of time and subsequent dose adjustments were made in order to avoid either toxicity due to high levels or possible organ rejection due to low levels of cyclosporine A. Results: 1) conversion of stable renal transplant patients from Sandimmune to Neoral requires a dose reduction to achieve similar trough levels; 2) a high, > 4 mg/kg, Sandimmune dose before conversion is most predictive of need for dose reduction. Converting from QD Sandimmune to BID Neoral is also predictive; 3) there is no statistical support for the hypothesis that African Americans and Whites differ in absorption of Sandimmune or Neoral. Both groups improved after conversion.